Femto cells are cost-active means of providing ubiquitous connectivity in broadband wireless networks. Femto is a prefix denoting a factor of 10−15 in the International System of Units. In telecommunications, a femto cell is a tiny cellular base station for use in home or small business. In the meantime, the 3rd Generation Project Partnership Long Term Evolution (3GPP LTE) standards specify evolved base stations, i.e., an evolved Node B (eNB) and a home eNB. The eNB corresponds to a normal macro base station and the home eNB corresponds to a femto base station.
FIG. 1 illustrates a sequence diagram for an inter-frequency handover procedure in the conventional wireless communication system.
In FIG. 1, a User Equipment (UE) 201 is operating on a first frequency band F1 within a serving cell under the control of a serving eNB 205. Reference numeral 203 denotes a second frequency band F2, and reference numeral 207 denotes a target eNB controlling a target cell to which the UE 201 to be handed over.
The UE 201 performs Inter-Frequency (hereinafter called as Inter-F) measurement (block 211). At this time, the serving eNB 205 of the UE 201 is operating on the first frequency band (F1), and the Inter-F measurement can be performed on the second frequency band (F2).
The UE 201 transmits a measurement report message to the serving eNB 205, and the measurement report is mainly triggered by two events: first the measurement on a neighbor cell is greater than the measurement of the serving cell over as much as a predetermined parameter value (offset) and, second, the measurement of the neighbor cell is greater than a predetermined threshold. Before starting measurement, the UE 201 can receive the parameter values, such as the offset and the threshold, through a measurement control message transmitted by the serving eNB 205.
If there is no allowed CSG cell and a range of Physical Cell ID (PCI) values reserved for use by valid CSG cells is known, the UE 201 does not performs inter-F measurements on the frequency bands of the PCI values reserved for use by the valid CSG cells. That is, the UE 201 performs the inter-F measurement on second frequency band (F2) of the public/macro cell (hereinafter called as macro cell).
Here, it is assumed that a cell B fulfills the Inter-F measurement report event on the second frequency band (F2) and the cell B is a macro cell on the second frequency band (F2).
Since the cell B fulfills the Inter-F measurement report event, the UE 201 transmits the measurement report message 221 including the frequency, PCI, and measurement about the cell B to the serving eNB 207.
Although the UE 201 has reported the measurement of the cell B, there may be a CSG cell having a good quality as compared to that of the cell B and fulfilling the inter-F measurement report event. For instance, it is assumed that a cell A on the second frequency band (F2) has a channel quality greater than the measurement of the cell B and fulfills the inter-F measurement report event, but is non-allowed CSG cell to access.
The serving eNB 205 received the inter-F measurement report makes a handover decision (block 231) to the cell B on the second frequency band (F2) and prepares the handover (block 233) by exchanging signals with the target eNB 207.
After the completion of the handover preparation, the serving eNB 205 transmits a handover command message 241 to the UE 201 instructing the UE 201 to hand over to the cell B.
Upon receipt of the handover command message, the UE 201 performs the handover (block 251) to the cell B on the second frequency band (F2). However, since the cell A is the best cell having the best radio channel quality, the UE 201 handed over to the cell B is likely to cause the inter-cell interference to other cell on the second frequency band (F2) including the cell A. In order to mitigate the inter-cell interference on the second frequency band (F2), the UE 201 attempts to hand over to the first frequency band (F1) (block 261). This causes the ping-pong effect and increases the signaling and processing overhead due to due to the frequent transmission of frequency measurement reports, resulting in degradation entire system throughput.